Abstract

The petrology and structural geometry of pre-granitic rocks in western Alpine County, California, were investigated by field and laboratory study. Pre-Cretaceous (Triassic?) metamorphosed equivalents of siltstones, sandstones, tuffs, limestones, andesite flows, and conglomerates are preserved in three roof pendants surrounded by a variety of Cretaceous (?) granitic rocks, principally adamellite and granodiorite.

The metamorphic rocks exhibit asymmetrical folds in the bedding (S1), the axes (βS1) of which trend northwest with an average plunge of 25°, and the axial surfaces of which dip steeply southwest. A statistically vertical tectonic surface (S2) trends northwest and varies about axes (βS2) approximately parallel to βS1 in some fields, and inclined to βS1 in others. Two sets of lineations are recognized, one parallel to βS1 (B) and another plunging steeply at approximately right angles to βS1 (B′). The geometric symmetry of both field and microscopic structures is triclinic. Deformation was probably by a combination of early flexural slip folding of S1 about an axis parallel to βS1, and later passive folding by slip on an intragranular scale along surfaces parallel to S2. The slip on S2 is thought to have been normal to the axis βS2 in the plane of S2. Strain of the mass involved shortening normal to the trend of the Sierra Nevada, accompanied later by extension in S2. A regional study suggests that the orientation and style of the structural geometry of deformed rocks are similar in many parts of the Sierra Nevada, particularly near the present crest of the range.

Post-kinematic thermal metamorphism caused recrystallization of most of the tectonites into hornfelses, some of which show relict textures. All the metamorphic rocks are in the hornblende hornfels facies. Three metamorphic assemblages are recognized which are characterized by (1) the pair calcite-amphibole, (2) clinopyroxene, and (3) wollastonite. The three assemblages are thought to be present because of gradients in composition and partial pressure of fluid phases during metamorphism. Metasomatically introduced boron, tungsten, molybdenum, iron, copper, tin, and sulfur are locally important components.

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